The need for low cost, compact and reliable optical detectors to perform optical sensing and monitoring is significant in optical communications networks to carry out the self-monitoring and diagnostic functions necessary to maintain quality of service. As the deployment of optical networking systems increases, the maintenance and trouble shooting of such decentralized systems becomes more challenging. To reduce network downtime and the costs associated with maintenance, there is an ever increasing need for optical components and subsystems which incorporate health monitoring functionality. Electronic systems already incorporate built-in monitoring and diagnostic capabilities to prevent catastrophic network failures. However, the realization of optical systems which perform this diagnostic and self-healing functionality requires the development of more compact, economical and more reliable optical detection and monitoring modules.
Costly fiber coupled power monitors have been deployed sparsely in long haul and metro networks to measure optical signal characteristics such as power, signal-to-noise ratio, and wavelength. Occasionally such monitors also receive low bandwidth supervisory data on radio frequency tones at 10 KHz, for example. The typical network monitor includes a tap splitter, either of the fused coupler type or micro-optic type, integrated with a discrete GaAs photodiode responsive to 1550 nm and 1310 nm wavelengths. The complexity of such a device results in significant cost and insertion loss. U.S. patent application Ser. No. 2004/0208442 by Shi et al., for example, describes an optical power monitor design based on a microoptic approach.
To monitor multiple dense wavelength division multiplexed (DWDM) channels within a single fiber, a diffraction grating demultiplexer and GaAs detector array has been the preferred approach. A scanning narrow band optical filter and photodetector may be used for lower performance applications. These monitors typically have response times of 1 to 100 ms.
A further limitation of present receiver technology is the difficulty in achieving a flat photodiode responsivity from dc to 10's of GHz. The response of high speed optical receivers typically exhibits a low frequency roll-off at 30 to 50 KHz due to the practice of applying a dc bias across the junction through an ac coupling circuit. Since some systems encode low frequency supervisory data of 10 to 20 KHz bandwidth, an inexpensive, low bandwidth optical detector to extract this supervisory data is of great value.
While optical detectors used in fiberoptic communication systems are generally restricted to photodiodes made of GaAs, Si and Ge, a wide variety of detector technologies have been developed for the general field of optics (see, for example, Infrared Detectors and Systems, Dereniak and Boreman, Wiley 1996). These detector types include photovoltaic, photoconductive, thermal, Schottky-Barrier photodiodes, multiple quantum well and superlattice detectors. A near universal characteristic of all these detector approaches is the very high optical absorption of the detector material at the wavelength of interest. These detectors operate in a mode wherein all light incident on the detector is absorbed. However, for optical power and signal monitoring applications, only a fraction of the power is available for detection. In these situations, present detector technology is inadequate.